1. Field of the Invention
The invention relates in general to a scanner and method thereof, and more particularly to a scanner with a driving device for feedback control and method thereof.
2. Description of the Related Art
Along with the advance in technology, scanners have become an indispensable image capturing device, while motor is an essential element for driving a photo sensing device to capture images. Direct current (DC) motor with the advantages of small size, low noise, low power consumption and low cost has now been widely adopted in a scanner.
Referring to FIG. 1A, a structural diagram of a DC motor based scanner according to prior arts is shown. Scanner 100 includes a scan flatbed 110, a chassis 120, a DC motor 130 and an application specific integrated circuit (ASIC) 140. The DC motor 130 drives the optical module 120 to move at a constant speed, meanwhile, the photo sensing device (not shown in the diagram) of the chassis 120, a charge coupled device (CCD) for example, captures images of scan lines of a to-be-scanned document 111 on the scan flatbed 110. The DC motor 130 has an encoder 132 for outputting a position feedback signal PF. The ASIC 140, according to the position feedback signal PF, controls the DC motor 130 to precisely position the chassis 120 to assure that the images captured are uniform in quality for each scan line.
However, due to the variation in speed control of the DC motor 130, the exposure time for different scan lines will not be the same. As shown in FIG. 1B, the scanner 100 uses the position feedback signal PF to determine the exposure time T1, T2, T3 of the photo sensing device sensing the to-be-scanned document 111. When the DC motor 130 drives the optical module 120 at a predetermined constant speed, the exposure time for reading a scan line is set to be T1=t2−t0. When the DC motor 130 drives the optical module 120 to sense a certain scan line at a speed higher than the predetermined speed, the exposure time for reading this scan line is set to be T2=t1−t0. When the DC motor 130 drives the optical module 120 to read a certain scan line at a speed lower than the predetermined speed, the exposure time for reading this scan line is set to be T3=t3−t0. For the exposure time T1, T2, T3 for reading different scan lines are not the same, according to the equation that exposure level=light density of the to-be-scanned document 111×responsivity of the photo sensing device ×exposure time×analog front end (AFE) gain, the exposure level for reading each scan line will be different and the quality of the captured image will be affected.
Referring to FIG. 1C, a circuit diagram of a DC motor based scanner disclosed in U.S. Pat. No. 6,037,584 is shown. Considering that the light density of the to-be-scanned document 111 and the spectral response of the photo sensing device are normally set to be constant, the patent adjusts the AFE gain dynamically to compensate the exposure time variation so that each scan line has the same exposure level.
Scanner 150 uses a DC motor 160 to drive an optical module 170 to scan the to-be-scanned document (not shown in the diagram). The DC motor 160 has an encoder 162 for outputting a position feedback signal PF to a timer 182 of an ASIC 180. An exposure control unit 184 controls the exposure time of the CCD 172 in the optical module 170 reading each scan line of the to-be-scanned document according to the position feedback signal PF. Data obtained by the CCD 172 as sensing the to-be-scanned document is output to an analog amplification unit 174 to be amplified. The ASIC 180 of the patent further includes a gain control unit 186, which is controlled by the exposure control unit 184 and the timer 182 to dynamically adjust the AFE gain of the analog amplification unit 174 to compensate exposure level variation of each scan line.
However, the scanner disclosed in above-mentioned patent needs to introduce an additional complicated circuit like the gain control unit 186 to the ASIC 180 in order to dynamically adjust the AFE gain, which makes the system more complicated.